The present invention relates to a method of adding a metallic additive to a molten metal having a temperature higher than the boiling point of the metallic additive.
In the manufacture of, for example, nodular graphite cast iron, magnesium having a relatively low solid solubility is added to a molten cast iron. It is well known that, during the manufacture thereof, the temperature of the molten cast iron is within the range of 1,300.degree. to 1,500.degree. C. on one hand and the magnesium to be added thereto has a melting point of about 651.degree. C. and a boiling point of about 1,110.degree. C. Because of this temperature difference, addition of magnesium to the molten cast iron under the atmospheric pressure results in that the magnesium is excessively excited to explosion. This is very hazardous to attendant workers.
Moreover, during the addition, magnesium particles to be added tend to be spattered over the ladle in contact with the molten cast iron and an insufficient amount of magnesium is consequently added to the molten cast iron. At the same time, since this addition causes the magnesium particles to transform from the solid state into the molten state absorbing heat energies evolved by the molten cast iron, the temperature of the molten cast iron tends to be lowered. In addition, in order for graphite to be sperodized, the magnesium particles must be uniformly dispersed in the molten cast iron during the manufacture.
Heretofore, the following methods are employed to add magnesium to a molten cast iron:
1. The intended addition is carried out within a protective container;
2. Magnesium to be added is introduced into the molten cast iron in an alloy or bricket form; and
3. Magnesium is added to the molten cast iron under the atmosphere wherein the pressure is higher than the vapor pressure of the magnesium.
With respect to the method (1) above, the method has been contemplated not only to minimize a possibility of danger by constructing a ladle in the form of a protective container, but also to cause the magnesium and the molten cast iron to uniformly contact to each other by the stirring action of the molten cast iron which results from explosion occuring upon introduction of the magnesium into the molten cast iron. In practice, however, this method has been found that minimization of the possibility of danger is insufficient and that addition of the magnesium to the molten cast iron often causes the temperature of the molten cast iron to decrease with a relatively large amount of the magnesium being ineffectively consumed.
With respect to the method (2) above, any of Ni-Mg alloy, Cu-Mg alloy and Fe-Si-Mg alloy is employed in place of pure magnesium. This alloy in a powdery form is, prior to being actually added to the molten cast iron, mixed with a binding agent to render it in the form of brickets or lumps. Although the use of the alloy or bricket is advantageous in that it can slowly be fused into the molten cast iron with successive explosion on small scale occurring therein so that the magnesium particles can uniformly be dispersed into the molten cast iron, the alloy itself is very expensive and, moreover, since the alloy or the bricket contains one or more substances other than magnesium and a relatively large amount of heat energies is required to fuse such substances in addition to that required to fuse the magnesium particles, the temperature of the molten cast iron tends to be lowered.
With respect to the method (3) above, the method has been contemplated to cause the added magnesium to be substantially completely fused into the molten cast iron without accompanying any explosion which may otherwise occur upon introduction of the magnesium into the molten cast iron. In practice, however, since there is a relatively great difference in specific weight between the magnesium and the molten cast iron, uniform distribution of the magnesium particles in the molten cast iron can hardly be achieved without the aid of an external mechanical stirring instrument or without a support for metallic additive, such as the magnesium, being moved up and down to stir the molten cast iron. In other words, the method (3) above requires for a ladle structure not only to be constructed in the form of a container which can withstand against the elevated temperature and considerably high pressure, but also to be equipped with an external stirring instrument. Accordingly, the ladle structure used in the practice of the method (3) above requires a complicated handling procedure and is often liable to troubles and, therefore, the method is not practically effectively carried out.